1. Field of the Invention
The present invention relates to a bearing construction which is particularly useful for mounting a crankshaft within an internal combustion engine.
2. Related Art
Internal combustion engines, particularly automotive internal combustion engines, typically use main bearing architectures in which a crankshaft is mounted to the cylinder block of the engine with bearing caps held in place with threaded fasteners. Conventional construction methods require that the cylinder block be machined separately to accept the bearing caps, which are themselves subject to separate manufacturing processes. Once both parts are machined, the cylinder block and bearing caps are joined together and then the main bearing bores are further machined, typically with boring and honing operations. Unfortunately, the necessity for separate bearing caps, produced by a completely separate manufacturing process, as well as the necessity for dowels and other devices to maintain the main bearing caps in proper alignment during the assembly of the engine and subsequent reassembly during service operations, renders the conventional block-and-cap construction both expensive and a burden to execute in high volume production.
An alternative method for producing bearing bores in a machine such as an internal combustion engine includes the use of fractured caps. In the fracturing process, the caps and the base of the bearing are cast as one piece. Then, following initial machining operations, the caps are fractured from the base, thereby producing a microscopically detailed parting line which promotes subsequent realignment of the caps upon their bases. Unfortunately, materials which are suitable for use with the fracturing process are frequently not optimal in terms of their physical properties. On point, it has been determined that a good deal of distortion occurs within main bearing bores in a firing engine when the fractured bearing caps are held in place with two fasteners. This distortion is shown in prior art FIG. 4 of this specification, in terms of a distorted main bearing bore experienced with the prior art bearing construction depicted in FIG. 6.
Although it would seem that additional stability could be imparted to the main bearing construction by making the fastener towers of the bearing cap larger, the fact is that a larger fastener tower would have the effect of reducing the contact force between the tower and the base, or cylinder block to which the cap is bolted, with the effect that fretting corrosion will occur as the bearing cap moves with respect to the cylinder block in response to firing pressure. This is a very undesirable situation. Another method to increase the bearing's strength is to add an additional fastener to the bearing caps so that four fasteners, instead of two, hold the bearing cap in place. This has the unfortunate effect of greatly increasing the cost in terms of materials and machining, as well as increasing the space required for the bearing cap, as well as its weight. All of these characteristics are undesirable.
It would be desirable to provide a fracture-separated main bearing cap which has strength characteristics approaching those of conventional caps made from materials which are not fractured from the bearing base, coupled with the ability to be fastened into an engine with only two fasteners.